I've read about this elsewhere ... what can be upgraded? I've checked summit and ecklers ... I don't see any overhaul kits with "better" hardware for higher-HP ...
Planning to overhaul the differential, but that's just replacing what's there ... seals/wheel bearings, 3rd member ... and of course, fresh gear oil. I don't remember ever having changed that!
You can't just bolt on Horse power. I'm building my engine from the ground up. I've got a 348 engine that I am going to stroke and bore for between 434-440 cid. Going to make it look like a 409 engine. You really just can't replace displacement unless you start going into forced induction.
You could do something similar with your car, but even though mid production a 409 became available I would make it look like the 350 HP 348 with triple deuces.
W engine performance builds though are not for the faint of heart. Number estimates keep climbing a little and by the time the engine is in the car and running I'll have close to 10 Grand in it. All the little things you forget when doing something like this adds up. New larger exhaust, accessory brackets, fan shroud, and more all cost money when they have 409 in their description.